The OP is asking an interesting historical question and I look forward to seeing some factual answers. The rules change from time to time, and after a few years it can be difficult to find out exactly what changes were made, and when.
We’ve seen the same kind of questions being asked on earlier threads, about how many days there are in Lent, and whether or not Sundays are counted. There is a historical reason for people’s doubts about that. When the forty-day Lenten fast was first introduced, during Constantine’s reign, the duration was exactly forty days, beginning on Quadragesima Sunday and ending on Holy Thursday. At a later date, the Church added four more days at the beginning of Lent, so that it then started on Ash Wednesday. Thus the total duration, instead of forty days, was now extended to forty-four days. That was when people started wondering whether they could legitimately leave out the Sundays, bringing the total down to forty days again, but of course, that didn’t work, because there are six Sundays in Lent, not four.